The invention concerns a brushless three-phase D.C. motor having a permanent-magnet arrangement and a three-phase winding, these being movable relative to each other, and having three position sensors which are stationary relative to the winding and controlled by the permanent-magnet arrangement to supply sensor output signals each of which is at a first potential for 180.degree.-el. but during the subsequent 180.degree.-el. at a second potential, the winding's coils being energized by current in a cyclical sequence in dependence upon the sensor output signals, which latter are offset by 120.degree.-el. one to the next; in the individual coils of the winding there are induced coil voltages offset by 120.degree.-el., the latter, in passing through zero, being alternately positive during an interval of at most 180.degree.-el. and negative during an interval of at most 180.degree.-el., their sum being equal to zero for all relative positions as between the magnet arrangement and the winding.
D.C. motors of this type in the form of rotary motors are known from Federal Republic of Germany "Offenlegungsschrift" DE-OS 31 22 049 and in the form of linear motors are known from Federal Republic of Germany "Offenlegungsschrift" DE-OS 31 23 441.
In the known motors the permanent-magnet poles are formed by a permanent-magnet ring having an approximately trapezoid-shaped radial-magnetization pattern or are formed by a succession of uniformly spaced, radially magnetized magnet segments, and the poles of the slotted flux-guide element are configured to be in cross section essentially T-shaped. The coils are in non-overlapping fashion each provided around one respective pole of a slotted flux-guide element. The ratio of the coil pitch to the magnet pitch amounts to 2:3. The slot openings between pole shoes of the slotted flux-guide element have a breadth between 3.degree.-el. and 30.degree.-el., whereas the magnet-pole breadth amounts to between at least approximately 120.degree.-el. and, at a maximum, 180.degree.-el. There are induced in the individual coils of the winding stepped voltages which are positive for about 120.degree.-el., almost zero for about 60.degree.-el., and negative for about 120.degree.-el. In the known motors the position sensors have been so arranged relative to the winding's coils that their change-of-state occurs substantially in the middle of the flank which the induced voltage exhibits in undergoing its transition from the 60.degree. zero-voltage interval to the positive interval. For this purpose the position sensors were either arranged at the middle of the slot openings between the pole shoes of the slotted flux-guide element or else at the symmetry axis off the poles of the flux-guide element. Connected between the outputs of the position sensors and an end stage serving to supply current to the winding, there was a decoder operating in such a way that, at any given time, two of the winding's coils were being energized by current at the same time that voltage was being induced in them in correspondence with the 120.degree.-el. intervals.
This known technique leads to a high degree of efficiency combined with simple winding design. However, a disadvantage is the fact that during the commutation interval the torque briefly drops to about 75%, or less, of the peak torque value. Faulty commutations, which are an important consideration in the case of small motors, can in practice lead to the torque's dipping to 50 to 60% of peak value. This is extremely undesirable in many applications.